The invention relates generally to current control and more specifically to a method and an arrangement for controlling a current through a winding interconnected in an H-bridge for controlling e.g. a stepping motor.
As is well known, in order to control the current through the winding to a desired value, such H-bridges are switched alternately during repetitive chopper periods among a magnetization state in which the winding is magnetized, a slow demagnetization state in which the winding is slowly demagnetized, and a fast demagnetization state in which the winding is quickly demagnetized.
If the desired value is constant or increasing, there are normally no problems to control the current to the desired value.
However, if the desired value decreases substantially, it is more difficult to control the current to the desired value.
A number of solutions to this problem are known.
The most common is to, in some way, sense whether the desired value increases or decreases. If the desired value decreases more than some specified criterion, the H-bridge is kept in the fast demagnetization state. Otherwise, it is kept in the slow demagnetization state. A disadvantage of this method is that a decision criterion for when fast demagnetization is to be used, has to be defined for each application and operational mode. This can be very time consuming. The method does not either solve resonance problems that arise when the actual value of the current oscillates.
According to another known method, an automatic choice is made between slow and fast demagnetization. In this case, a clock pulse is used, which always switches the H-bridge to the magnetization state. Just before, during or just after the switching of the H-bridge to the magnetization state, a comparison is made between the actual value and the desired value of the current through the winding. The result of this comparison controls in its turn whether fast or slow demagnetization is to be used after the magnetization phase. The actual value of the current through the winding increases during the magnetization phase. The actual value is compared to a desired value and when the actual value reaches the desired value, the H-bridge is switched to either the fast or the slow demagnetization state, the choice being controlled by the previous current comparison. The choice between the two demagnetization states can then be varied in accordance with a couple of different methods up to the next clock cycle when magnetization again is initiated.
The disadvantage of this method is that a relatively large current ripple is obtained due to the fact that there is a short period during each clock cycle when magnetization is carried out independent of both the actual value and the desired value. By also controlling the time during which fast demagnetization is used, the ripple can be improved to a certain extent by that method. However, a requirement is that it should be possible to measure the current through the winding irrespective of the state of the H-bridge. In practice, this is quite difficult since the current has to be measured in series with the winding. The potential in the point where the current measurement has to take place, varies strongly. Thus, this measurement method is rarely used.
As examples of the prior art, U.S. Pat. No. 4,908,562 and Swedish Patent Application No. 9800131-6 can be mentioned.